The program has been developed in order to examine the major physicochemical factors involved in intercellular and cell-media interactions. It is proposed to evaluate the relative importance of the state of 'aggregation' or association of the blood elements, their flexibility and electrokinetic charge as factors which influence the flow properties of the system. The rheological behavior of native and aldehydefixed (hardened) human red blood cells suspended in electrolytes and media containing various concentrations of plasma proteins, hydroxyethyl starch or dextrans will be examined over a range of shear rates by rotational viscometry. The degree of aggregation of cells under comparable conditions will be assessed by light microscopy and the electrokinetic charge of the cells will be obtained from measurement of their electrophoretic mobility. The deformability of the cells will be determined by centrifugal packing and also by a screen filtration pressure technique. Microscopically red cell aggregation or disaggregation are mirrored rheologically in the relative and reduced viscosities of erythrocyte suspensions obtained at low shear rates. These observations will be correlated with the electrokinetic charge of the cells and these data analysed in order to test the adequacy of colloidal stability theory in describing the interactions which occur between cells in suspension. The studies will ultimately be extended to whole blood so that a comparison may be made between the behavior of bloods obtained from normal subjects with those from patients with cardiovascular disease.